By the looks of him I'd say he is close to 6+ months old.. he looks full grown to me.. judging by the heater and plants in the background. He doesn't look super old but does look fully grown. That being said bettas can live up to 7 years with the right care so you should still have a long life with him.

With regards to size, he looks deceptively big in the pics. I would say in reality he is about 60% the size of full grown veil tails that come in a cup. I am asking if he will get bigger as I do not know if all Bettas grow to be about the same size or if he is of some smaller breed of Betta that does not grow large. (pardon my ignorance).